In the oil and gas industries, hydrocarbons are obtained from deep in the earth by drilling wells into the ground to access the hydrocarbons. The hydrocarbons are contained in pores in permeable rock which is situated deep in the ground which must be drilled through in order to access the hydrocarbons. Following the drilling phase, the well is clad with a metal casing in order to support the rock and prevent the hole from collapsing. An additional metal tube which is smaller in diameter than the casing, usually of a fixed diameter, provides a conduit to contain the hydrocarbons in a pressure tight environment from near the bottom of the well to the surface.
At the surface, the metal tube (known as tubing) is terminated in a well head which features a number of valves to allow pressure and flow control of the hydrocarbons. At the bottom of the well, a packer prevents pressure from entering the void between the casing and the tubing. The packer is usually conveyed into the well along with the tubing during the well construction phase.
In the past, most wells have been drilled vertically, but technology now allows the drilling of what is termed “horizontal” wells. Wells are often deviated from a vertical direction to a high angle in order to access a large area from a central drilling point or to access remote pockets of hydrocarbons. Development of this technology has allowed horizontal drilling to evolve whereby the well is deliberately angled at up to 90 degrees when it passes through the hydrocarbon bearing rock, in order to maximise the contact area between the well and the hydrocarbon producing area. Not only does this improve the productivity of the well, but it increases the effective drainage area where the well is positioned.
One problem associated with operating at high deviations, or at 90 degrees to the vertical, is the absence of gravity to assist the process of lowering tools and instruments into the well. During the drilling phase, this is not a problem since the pipe which is used to drill the well may be pushed down the well and into the deviated section. During the latter stages of well construction, temporary pressure barriers are placed and removed using wireline techniques whereby tools and equipment are lowered down the well and positioned on the end of a wire. The wire may be of two types, slickline or electric line, both of which are spooled on a drum which may be rotated in and out. The tools are conveyed into the well assisted by gravity only, and will halt when a certain angle of deviation is reached, normally somewhere between 65 and 75 degrees.
Packers which are conveyed into the well on the tubing usually require the end of the tubing to be closed off so that pressure may be applied internally to set the packer. This also serves the purpose of checking the pressure integrity of the tubing before production start up. Occasionally, this pressure barrier is left in the well for some time to allow commissioning work to be undertaken at surface or on other adjacent wells. During this time, a drilling rig may be repositioned or removed for operational reasons. Normally the pressure barrier (also known as a plug) is removed using wireline techniques.
It is advantageous for the temporary pressure barrier to be left downhole but opened or bypassed by being operated remotely from the surface and without any sort of well intervention. This option is especially attractive if either it is in a highly deviated section of the well, if the well has been suspended for some time, or if the well is a sub-sea completion and no surface facilities exist. Additionally, such devices remove the requirement for well intervention in normal wells, thus saving time and cost.
A number of devices exist which provide this operational functionality. For example, pressure barriers may take the form of a ball valve, a glass disc or more recently a solid plug of a salt and sand compound. Actuation of all of these may be performed by a repeated pressure cycling to stress, and ultimately break, a retaining member, by application of pressure to overcome a shear disc or shear pins, initiation of a small explosive charge following recognition of an applied pressure signal, multi pressure cycles advancing a ratchet mechanism to allow actuating pressure ingress, or a combination of more than one of these. A wide range of actuation methods and procedures have been established in a variety of other downhole tools.
One type of plug apparatus is disclosed in U.S. Pat. No. 6,076,600, and relies on fresh water stored in the tool contacting a plug compound consisting of sand and salt, and dissolving the salt element following the actuation process. This tool has the advantage that the salt plug disappears following correct operation, but is easily disposable in the event of failure. However, this tool is also prone to failure through insufficient dissolution of the salt plug by the fresh water stored in the tool.
In addition, all of the above devices suffer from the disadvantage that failure of operation requires intervention into the well to remedy the problem, which is usually expensive, as a rig is usually required and time will be spent not only in the remedial work, but in mobilising and demobilising the rig. In addition, some of the above systems have been found to partially function, and in the case of ball valves, to only partially open. This provides the further disadvantage of constricting the flow and may also prevent access to a lower section of the well at a later date. Furthermore, a complete failure of a ball valve whereby it fails to open requires that the ball be milled out, which is a very expensive and time consuming operation which also threatens the integrity of the well, and is therefore to be avoided.
Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art.